Coating for aqueous inkjet transfer

ABSTRACT

An image transfer member (ITM) is provided with a skin over its surface in which the skin incorporates a hygroscopic agent. The hygroscopic agent is operable to make the ITM skin very hydrophilic or to provide a high surface energy. One such hygroscopic agent is glycerol that may be applied to the surface of the ITM with a carrier, such as water. In an image transfer process, the carrier is completely or partially removed, such as by drying, leaving a thin skin of the hygroscopic agent. Ink drops applied in an image pattern onto the skin spread without puddling or draw-backs, producing an optimum wet image.

CROSS REFERENCE

This application cross-references the following co-pending U.S. patentapplications, all of which were filed on Sep. 20, 2013, and the contentsand disclosure of which are incorporated herein by reference:

Ser. No. 14/032,945, entitled “SYSTEM AND METHOD FOR IMAGE RECEIVINGSURFACE TREATMENT IN AN INDIRECT INKJET PRINTER,” filed on Sep. 20,2013;

Ser. No. 14/032,996, entitled “IMPROVED COATING FOR AQUEOUS INKJETTRANSFER”, filed on Sep. 20, 2013; and

Ser. No. 14/033,042, entitled “IMPROVED COATING FOR AQUEOUS INKJETTRANSFER”, filed on Sep. 20, 2013.

TECHNICAL FIELD

present disclosure is generally directed to imaging transfer systems,particularly systems using aqueous ink. More specifically, thedisclosure concerns improved coatings for image transfer surfaces.

BACKGROUND

An image transfer system includes an intermediate transfer member (ITM)to initially receive an image transferred by a plurality of ink jets, asdepicted in the diagram of FIG. 1. The ink jets are configured totransfer an aqueous ink onto the surface of the transfer member in avariety of known manners. The aqueous image is partially dried beforereaching the transfer roll. A substrate is pinched between theintermediate transfer member and the transfer roll and the ITM releasesthe ink image onto the substrate in a transfer step. The substrate isconveyed to post-processing components that fix the image onto thesubstrate. It is understood that the ITM and transfer roll arecontinuously rotated and that a substrate or substrates are continuouslyfed through the transfer system and between the ITM and transfer roll.Image transfer systems of the kind generically depicted in FIG. 1 areused in a wide range of machines, such as printers, copiers, facsimilemachines, book making machines and the like.

The surface energy of the surface of the ITM controls how well the inktransferred to the ITM surface is retained on the surface and how wellthe ink image is released from the ITM onto the substrate. The problemsof ink retention and release are exacerbated in high-throughput systemswhere the substrate is fed at high speeds through the image transfersystem. A low surface energy is desirable for optimum image transferfrom the surface of the ITM to the surface of the substrate. On theother hand, a low surface energy reduces the ability of the aqueous inkto spread on the ITM surface, resulting in a low image quality. Aqueousink jet imaging on a low surface energy, non-absorbing surface and thenoptimal release and transfer to the substrate has been very challenging,with no commercially viable solution thus far.

An optimum surface treatment for an ITM must tackle three challenges: 1)wet image quality; 2) image transfer; and 3) print-head management. Thefirst challenge—wet image quality—prefers a high surface energy on theITM surface which causes the aqueous ink to spread and wet the surface,rather than beading up into discrete droplets. The secondchallenge—image transfer—prefers that the ink, once partially dried onthe ITM, has minimal attraction to the ITM surface so that 100% of theink is transferred from the ITM to the substrate. Thus, image transferis optimized by minimizing ITM surface energy. The third challengerelates to how well the print head can be kept clean of dried ink. Forresin-based ink, the drying of the ink on the face plate of a print headcan render it inoperable. On the other hand, too much moisture cancondense on the face plate and cause jetting problems. In addition, someink jets can be sensitive to high temperatures, typically temperaturesabove about 70° C.

Various approaches have been investigated to provide a solution thatbalances all three challenges, including ITM material selection, inkdesign and auxiliary fluid methods. With respect to ITM materialselection, materials that are known to provide optimum releaseproperties include the classes of silicone, fluorosilicone, TEFLON,VITON and certain hybrid materials. These compositions have low surfaceenergy but provide poor in wetting. Alternatively, polyurethane andpolymide have been used to improve wetting but at the cost of poor inkrelease properties. Tuning ink compositions to address these challengeshas proven to be very difficult since the primary performance attributeof the ink is the performance in the print head. For instance, if theink surface tension is too high it will not jet properly and it if istoo low it will drool out of the face plate of the print head.Compounding the problem is the fact that for optimal ink transfer, inkcohesion must be significantly greater than the ink-to-ITM adhesion forall image contents, including the stress cases of single layer small dotand three layer process black solid printing.

The wet image quality is directly affected by surface energy of the ITM.As mentioned above, low surface energy is typically necessary for imagetransfer, but this same low surface energy property diminishes theability of conventional inks, and more particularly aqueous inks, tospread on the ITM surface. When ink drops coalesce at a state ofinsufficient spreading, inks from multiple drops can reflow orredistribute in many undesired ways, which ultimately produce imagedefects. The problem of poor-coalescence is enhanced on non-absorbingsubstrates because there is no simple mechanism to freeze the motion ofthe colorant in the ink drops. Examples of the poor wet image qualityresulting from low surface energy are shown in FIGS. 2a, 2b . In theexample of FIG. 2a , the ink drops forming the figures “c”, “q” and “6”puddle resulting in a very poor image quality. In the example of FIG. 2b, the lines of ink should be 100% continuous at 600 dpi with a 12 pldrop size. However, as the picture illustrates, the ink drops draw backdue to poor coalescence to that the lines are sporadic and incomplete,again resulting in a poor image quality. Poor ink spreading is the causeof the poor quality in both examples.

There is a need for an ITM that can maintain a desirably low surfaceenergy density, for image transfer, but can also promote optimum inkspreading.

SUMMARY

In one aspect, an ink spreading enhancing coating is provided for anindirect image receiving member or blanket in an aqueous printing systemin which the coatings include a hygroscopic composition and asurfactant. One improved coating composition includes a glycerolcomposition in a liquid carrier as the hydroscopic composition. Theimproved hygroscopic coating composition is applied to the surface ofthe blanket and partially dried before the aqueous ink is applied. Inkapplied in an imagewise manner (i.e., according to an image transmittedto the printing device) is at least partially dried prior to reachingthe transfer station where the ink image is transferred to a substrateconveyed between the blanket and a transfer roll. The surface of theblanket is then cleaned of any residual hygroscopic coating and ink (ifpresent) and the blanket surface continues to the next application ofthe improved starch coating composition.

In one aspect, a coating with improved transfer performance is providedfor an indirect image receiving member or blanket in an aqueous printingsystem in which the coatings include a hygroscopic composition, at leastone binder composition (such as starch) and a surfactant. The improvedhygroscopic coating composition is applied to the surface of the blanketand at least partially dried before the aqueous ink is applied. Inkapplied in an imagewise manner (i.e., according to an image transmittedto the printing device) is at least partially dried prior to reachingthe transfer station where the ink image is transferred to a substrateconveyed between the blanket and a transfer roll. The surface of theblanket is then cleaned of any residual of the hygroscopic/bindercomposition and ink (if present) and the blanket surface continues tothe next application of the improved starch coating composition.

DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of components of a conventional image transfersystem.

FIG. 2a is a micrograph of ink drops applied to a low surface energy ITMdepicting puddling of the drops.

FIG. 2b is a micrograph of ink drops applied to a low surface energy ITMdepicting draw-back of the drops due to coalescence.

FIG. 3a is a micrograph of ink drops of three volumes (5 pl, 7 pl and 12pl from left to right) applied to an ink-jet premium photo paper.

FIG. 3b is a micrograph of ink drops of three volumes (5 pl, 7 pl and 12pl from left to right) applied to a skin containing glycerol that hasbeen coated and partially dried on an ITM.

FIG. 4a is a micrograph of ink drops of three volumes (5 pl, 7 pl and 12pl from left to right) applied to a skin that has been coated andpartially dried on an ITM, the skin composition has a glycerol-binderratio of 1:1.

FIG. 4b is a micrograph of ink drops of three volumes (5 pl, 7 pl and 12pl from left to right) applied to a skin that has been coated andpartially dried on an ITM, the skin composition has a glycerol-binderratio of 2:1.

FIG. 5 is a diagram of an image transfer system, such as the system ofFIG. 1, modified for application of an improved coating compositiondescribed herein.

DETAILED DESCRIPTION

The present disclosure relates to image transfer systems fortransferring an ink image onto a substrate, and particularly to acoating composition for an image transfer surface. As described hereinthe image transfer surface is part of an intermediate transfer member(ITM) that is adapted for continuous passage of a substrate across theimaging surface of the ITM. It should be understood, however, that otherimage transfer systems may benefit from the improved coatingcompositions disclosed herein. It should also be understood that theimage transfer systems may be part of an imaging or printing machine,such as a printer, copier, facsimile machine, bookmaking machine, andother machines operable to apply an image to a substrate.

In one aspect of the disclosure, a “skin” is applied to the surface ofthe ITM that is hydrophilic to assist in spreading of ink drops in theprinting process. As used herein, a “skin” is a layer that is applied tothe surface of the ITM that is optionally dried to form a skin surfaceonto which ink can be dispensed. As used herein, the term “hydrophilic”refers to the ability to attract water molecules or other solvents usedin aqueous ink. In accordance with the disclosure, a hygroscopic agentis applied to the surface of the ITM to form the skin. The hygroscopicagent may be combined with a carrier to facilitate application to theITM surface. The carrier is completely or partially removed once thehygroscopic agent-carrier composition has been applied to the surface ofthe ITM to form a thin skin onto which the inkjet image is applied. Thecarrier thus has an evaporation temperature, or boiling point, that iswithin the range of typical process temperatures utilized in imagetransfer machines. Moreover, the hygroscopic agent has an evaporationtemperature that is greater than the evaporation temperature, or boilingpoint, of the carrier so that the agent remains after the carrier hasbeen removed to form the skin on the ITM. The hygroscopic compositionmay be applied to the surface of the ITM using the system shown in FIG.5, as described in more detail herein.

The hygroscopic agent may be selected from several materials, such assugar, caramel, honey, syrup, glycerol, and ethylene glycol. A suitablecarrier is water, particularly for an image transfer machine thatutilizes aqueous ink since the process temperatures of the machine willbe calibrated to remove the water in the applied ink.

One hygroscopic agent found to provide good wet image quality isglycerol. Glycerol is well-suited for this application because it ischemically pure, has a high viscosity (1.412 Pa·sec) and high boilingpoint (290° C.) and a very high surface tension (64 dynes/cm). In onecomposition, glycerol is combined with water as a carrier to form a2-10% solution. During the skin drying process, some or all of the wateris removed by heating leaving an inkjet imaging skin having a thicknessof about 0.05-1.0 microns, preferably 0.1˜0.3 microns. Alternatively,the hygroscopic composition may contain no substantial amount of carriersuch as water. No skin drying is required for this composition oncecoated onto the ITM. The skin formed by this alternative composition mayhave a thickness of about 0.2˜2.0 microns, preferably 0.2˜0.5 microns.

The process of ink drop spreading or draw-back is well-known. Typically,upon impact on a substrate, the ink drop spreads to a spot size quicklyby pure mechanical momentum, which can be called the impact spot size.The subsequent changes in spot size are strongly influenced by surfaceenergy of the substrate and the surface tension of the ink and can occurin three different ways: 1) the spot size will not change, which iscalled pinning; 2) the spot size will decrease, which is calleddraw-back; 3) the spot size will further increase, which is calledspreading. On a low surface energy ITM, strong draw-back occurs. Tofight this draw-back problem, prior-art indirect aqueous inkjet printingsystems and methods provided various ways to pin the drops. However,pinning only allows a maximum drop size that is close to the impact dropsize. For image quality and cost concerns, there is a strong desire tohave more ink drop spreading that can produce spot sizes significantlygreater than the impact spot size. This is particularly important forsmaller drop sizes since the impact momentum is small and the impactspreading is less effective.

The effect of the glycerol skin on ink drop spreading is illustrated inmicrographs of FIGS. 3a-3b . FIG. 3a depicts a pattern of three inkdrops (5 pl, 7 pl and 12 pl from left to right) applied to a inkjetpremium photopaper, with the mean effective diameter of the drops afterspreading. Inkjet photo paper is a specially engineered quick absorptionpaper. Ink drops are quickly absorbed shortly after the initial impact.The spot size is a fair indication of the impact spot size. Inparticular, the 7 pl ink drops on the inkjet premium photo paper in themiddle of FIG. 3a spread to a diameter of about 38 microns, which is notsufficient for a 600 dpi printing. The same three drops on the bare lowsurface ITM would have produced even smaller spots due to draw back (notshown). The same three ink drops were applied to the glycerol skincoated on an ITM as described above. With the same volume of ink drops(7 pl), the drops spread to diameters that were over twice as large aswith the photo paper, which will be more than sufficient for 600 dpiprinting. For instance, the left-most 5 pl drops that had a 29 microndrop diameter without the skin yielded a 76 micron diameter with thehygroscopic skin. For the right-most 12 pl drops in FIGS. 3a, 3b thespread diameter increased from 52 microns to 118 microns.

The hygroscopic skin disclosed herein is demonstrated to dramaticallyimprove the drop spreading and increase the spot size significantlybeyond the impact spot size. Thus, the present hygroscopic compositionyields a spot size that is greater than the impact spot size in aninkjet printing system, including spot sizes that are at least 1.2 timesgreater and more than 2 times (twice) the impact spot size.

The hygroscopic skin disclosed herein improves the ink spreading even ona low surface energy ITM surface because the skin acts as a barrierbetween the applied ink and the ITM. Since the hygroscopic skin producesbetter ink spreading a smaller ink drop can be used to produce the sameimage quality that can be achieved with an untreated ITM surface. Inparticular, while a typical photographic paper requires the inkjet todispense the ink in 12 pl drops, the hygroscopic skin allows a 5-7 pldrop to produce the same surface spread and provide the same or betterresults for 600 dpi image transfer processes. Smaller ink drops meansreduced ink usage which ultimately leads to a reduction in cost.Moreover, smaller drops mean that less water needs to be dried beforetransfer to paper.

The hygroscopic agent may be combined with other materials acting as ahydrophilic binder or a hydrophilic polymeric agent to help hold theskin to the surface of the ITM, improve the structural strength of theskin and improve the transfer performance. One such hydrophilic agentmay be starch combined with the hygroscopic agent-carrier solution. Inone example the agent is glycerol and the carrier is water provided inthe 2-10% solution described above. The impact of the addition of thebinder to the glycerol-water solution is illustrated in the micrographsof FIGS. 4a-4b . The solution in FIG. 4a includes starch in 1:1 ratiowith the glycerol, while the solution in FIG. 4b is 2:1 glycerol tostarch. It can be seen that the 2:1 composition produced larger spreaddiameters, but in all cases the diameters with the modified skin weregreater than the standard photographic papers illustrated in FIG. 2a .Other agents may be added to the skin composition, such as a surfactantto improve various functions of the composition, in particular toenhance the coating coverage of the hygroscopic composition on the ITM.

The hygroscopic agent and carrier composition may be provided forapplication to an ITM 10 as depicted in FIG. 5. A coating applicationelement 12 may be an Anilox roll configured to apply the hygroscopicagent-carrier solution in a known manner to the surface of the ITM 10.The solution is at least partially dried by a conventional dryingelement 14, which reduces the thickness of the skin to a thickness ofbetween 0.1-0.3 microns. The skin is advanced to the ink jet station 16where aqueous ink is jetted onto the film in an imagewise manner (i.e.,according to an image transmitted to the printing device). The ink imageis dried at a drying station 18 prior to reaching the image transferstation 20 where the substrate passes between the ITM and the transferroll, where the ink image is transferred to the substrate. The skinitself may also be transferred with the ink image. Once the ink imagehas been lifted from the ITM and transferred onto the substrate, the ITMcontinues to the cleaning station 22 where the surface of the ITM iscleaned of all liquid, residual ink and other debris in a known manner.The ITM surface is then ready for another application of the hygroscopicagent composition disclosed herein.

In the present disclosure the hygroscopic agent has been described asbeing applied as a layer onto the surface of the ITM, with the ink beingapplied directly onto the hygroscopic layer. Alternatively, thehygroscopic agent may be applied to augment or to act as a precursor toa hydrophilic agent applied to the surface of the ITM. The hydrophilicagent can be a starch composition as described in co-pending applicationSer. No. 14/032,996, incorporated by reference above, or a polyvinylacetate (PVA) composition as described in co-pending application Ser.No. 14/033,042, incorporated by reference above. The hygroscopic agentmay be applied over a layer of the hydrophilic agent to enhance thehydrophilic action of the coating. Alternatively, the hygroscopic agentmay be added as an ingredient to a coating composition formed with thestarch or PVA compositions. The addition of the hygroscopic agent canboost the wetting or spreading capacity of the coating composition onthe ITM. This enhanced wetting capability may be particularly useful forinks with relatively high surface tension that experience difficulty inspreading on the ITM or the hydrophilic composition alone.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Variouslypresently unforeseen or unanticipated alternatives, modifications,variations or improvements therein may be subsequently made by thoseskilled in the art which are also intended to be encompassed by thefollowing claims.

What is claimed is:
 1. A composition for forming a skin on the surfaceof an image transfer member in an aqueous ink imaging system,comprising: a carrier in a mixture with a hygroscopic agent, wherein thecarrier is water and the hygroscopic agent is glycerol in a 2-99% (twoto ninety-nine percent) mixture with the carrier; and a hydrophilicagent.
 2. The composition of claim 1, wherein the hygroscopic agent andthe hydrophilic agent are provided in at least a 1:1 ratio ofhygroscopic agent to hydrophilic agent.
 3. The composition of claim 2,wherein the hygroscopic agent and the hydrophilic agent are provided inat least 2:1 ratio of hygroscopic agent to hydrophilic agent.
 4. Thecomposition of claim 1, wherein the hydrophilic agent is a starch. 5.The composition of claim 1, wherein the hydrophilic agent is a polyvinylacetate composition.
 6. An aqueous ink transfer process comprising:applying a mixture to the surface of an image transfer member (ITM), themixture including a hygroscopic agent and a carrier; partially dryingthe mixture to at least partially remove the carrier, leaving an imagingskin consisting essentially of the hygroscopic agent on the surface ofthe ITM; applying aqueous ink onto the skin; partially drying the ink;and transferring the ink onto a substrate.
 7. The aqueous ink transferprocess of claim 6, wherein the hygroscopic agent is selected from thegroup of sugar, caramel, honey, syrup, glycerol, and ethylene glycol. 8.The aqueous ink transfer process of claim 6, wherein the carrier iswater.
 9. The aqueous ink transfer process of claim 8, wherein thehygroscopic agent is glycerol in a 2-10% (two to ten percent) mixturewith water.
 10. The aqueous ink transfer process of claim 6, wherein:the hygroscopic agent has a boiling point that is greater than theboiling point of the carrier; and the first drying step is at atemperature between the respective boiling points of the hygroscopicagent and the carrier.
 11. The aqueous ink transfer process of claim 6,wherein: the aqueous ink is applied at an impact drop size; and the inkdrop spreads on the skin to a drop size that is greater than the impactdrop size.
 12. The aqueous ink transfer process of claim 6, wherein theink drop spreads on the skin to a drop size that is at least 1.2 timesgreater than the impact drop size.
 13. The aqueous ink transfer processof claim 6, wherein the ink drop spreads on the skin to a drop size thatis at least two (2) times greater than the impact drop size.
 14. Theaqueous ink transfer process of claim 6, wherein the skin has athickness of about 0.05-1.0 microns.